Telomerase is a reverse transcriptase that synthesizes telomere repeats and is reactivated in 90% of human cancers. Extension of telomeres by reconstitution of telomerase in human cells in culture prevents senescence and crisis responses caused by telomere shortening, leading to cellular immortalization. This ability to maintain telomeres and prevent checkpoint activation is likely important in the role of telomerase in facilitating carcinogenesis. Recent observations, however, suggest that the ability of telomerase to support proliferation and tumorigenesis extends beyond its activity in preventing critical telomere shortening, although the mechanisms by which telomerase serves these functions and the role of telomerase in normal development and homeostasis remain obscure. To better understand these functions, we have generated transgenic mice in which expression of both components of the telomerase ribonucleoprotein (RNP) are tetracycline-inducible. The mouse is an ideal model system in which to study these novel activities of telomerase because telomeres are sufficiently long that critical telomere shortening is not seen. Here, we show that conditional activation of the telomerase RNP in vivo encourages normally quiescent epithelial cells to proliferate, severely disrupting kidney function and causing rapid, premature death from kidney failure. We hypothesize that during development telomerase is downregulated to allow cell cycle exit and terminal differentiation and that reactivation of telomerase, in cancer or in telomerase RNP transgenic mice, prevents these processes. Our specific aims are as follows: 1) To better define the mechanisms by which activation of the telomerase RNP perturbs epithelial homeostasis. We will determine if the effects of telomerase on cellular proliferation are cell autonomous and if they require enzymatic activity. Extinguishing telomerase activity will enable us to distinguish between models of telomerase action. 2) To determine how telomerase enhances proliferation, blocks differentiation and enhances self-renewal in tissue culture-based model systems. The impact of telomerase activation on diverse cell types will be tested in several tissue culture -based systems. Mechanisms of telomerase in enhancing proliferation and in blocking differentiation will be explored. 3) To analyze the impact of conditional deletion of telomerase in mouse tissues and organs. Through analysis of a conditional knockout mouse, we will determine the role of endogenous telomerase in cells in culture and in stern/progenitor compartments in vivo.